Light Detection and Ranging (LIDAR) is a remote sensing method in which lasers are used to probe, range, or image a scene. LIDAR works by transmitting laser light toward an object in a medium. The laser light is reflected off the object and the reflected signal is captured by a photodetector. The detected reflection is processed and used to create an image of the target object.
In degraded visual environments (e.g., fog, smoke, haze, turbid water, etc.), the accuracy of standard LIDAR systems diminish as the visibility of the target object is reduced due to the collection of scattered light. Backscattered light from the environment, having never reached the object of interest, reduces image contrast and increases receiver noise. In the direction of propagation, small-angle forward scattered light broadens the interrogating beam in space and time, leading to reductions in both spatial and range resolution. Depending on the severity, the interrogating beam may be spatially dispersed enough to illuminate parts of the scene outside of the area of interest, further degrading resolution.
Advances have been made in LIDAR imaging to compensate for the decreased contrast and loss of resolution caused by optical scattering by particles in the medium. The simplest approach modifies the system geometry. Increasing the separation distance between transmitter and receiver can reduce backscatter clutter—at the cost of a larger system size—and a reduced receiver field of view can limit forward scatter clutter—at the cost of reduced signal level. Alternatively, the temporal properties of the laser pulse can be exploited to selectively process only the reflections from the target object, for example through receiver gating. This helps reduce backscatter, but makes the system blind to nearby targets since the receiver is gated off until the anticipated arrival of longer range returns. Furthermore, receiver gating cannot discriminate against forward scatter. In another variation, an intensity modulated beam—or an intensity modulated pulse—has been shown to effectively reduce both backward and forward scatter. Currently however, this approach requires large and complex laser sources.